Pulse compression is an established technique for generating optical pulses shorter than those produced directly by lasers or amplifiers. Most commonly, additional bandwidth is generated by self-phase modulation (SMP) as the pulse propagates nonlinearly in an optical fiber. The negative group-velocity dispersion (GVD) required to compress the pulse is typically provided by gratings or prisms. Compressors based on single-mode fibers are limited to nanojoule pulse energies by higher-order nonlinear effects, and ultimately by damage to the fiber. Thus, new approaches are needed for compression of the high-energy pulses that are now readily available from chirped-pulse amplifiers, for example.
Bulk materials can be used for pulse compression. However, several third-order nonlinear-optical processes occur when high-energy femtosecond-duration pulses interact with a solid. The output beam typically has different frequencies propagating in different directions, and is difficult to control. As a consequence, the use of bulk third-order materials for pulse compression has not found significant use.
One possible solution to this problem was reported by Nisoli et al. These workers achieved large spectral broadening by propagating pulses through a high-pressure noble gas confined in a hollow-core waveguide of fused silica. Excellent results were obtained, including compression from 140 to 10 fs. Pulse energies as high as 240 xcexcJ were produced with 660-xcexcJ input pulses. Although the compressed pulse energy is a substantial improvement on that achievable with ordinary fibers, these results do point out a limitation of this approach: because the pulse does not propagate as a guided mode, the waveguide is lousy. Additional drawbacks include the susceptibility of the waveguide to optical damage, the complexity associated with handling the high-pressure gas, and a lack of commercially-available components.
Recent work has shown that second-order nonlinearities can be exploited for pulse compression. Following on work by Wang and Luther-Davies, Dubietis et al. Demonstrated that pulses can be compressed in phase-matched type-II second-harmonic generation. This approach relies on group-velocity mismatches (GVM) among the three interacting waves, and requires division of the input pulse into the o- and e-wave components needed for the type-II process as well as an appropriate pre-delay of one of the input fundamental pulses. Compression from 1.3 ps to 280 fs was demonstrated, with energy conversion efficiency of close to 50%. Dubietis and co-workers have also demonstrated the phase-matched generation of second-harmonic pulses shorter than the input fundamental pulse through pulse tilting. Here we show that negative phase shifts produced in phase-mismatched type-I second-order processes can be exploited for effective pulse compression. Our approach is conceptually similar to that employed in traditional compressors: in a first stage the pulse accumulates a nonlinear phase shift, and the pulse is then compressed by dispersive propagation in a second stage. Positive GVD is needed for compression, and this can be provided by a suitably-chosen piece of transparent material. 120-fs pulses are compressed by a factor of 4, and higher compression ratios should be possible. The compressor is efficient, with the compressed pulse amounting to at least 85% of the input-pulse energy.
It has been known for years that the cascading of "khgr"(2)(xcfx89;2xcfx89,xe2x88x92xcfx89) and "khgr"(2)(2xcfx89;xcfx89,xcfx89) processes leads to a nonlinear phase shift xcex94"PHgr"NL in a pulse that traverses a quadratic medium under phase-mismatched conditions for SHG or parametric processes. The phase shifts can be either positive or negative, depending on the sign of the phase mismatch xcex94kL (xcex94k=k2xcfx89xe2x88x922kxcfx89). Bakker and co-workers performed a theoretical study of the phase shifts generated by three-wave interactions, and large cascade nonlinear phase shifts were later measured in KTP and periodically poled LiNoO3 (PPLN).